The present invention relates broadly to a signal detection circuit and in particular to a digital detection apparatus for bi-phase signals.
The state of the art of signal detection and recovery apparatus is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. patents: U.S. Pat. No. 4,531,223 issued to Ashida on 23 July 1985; U.S. Pat. No. 4,468,791 issued to Masek on 28 August 1984; U.S. Pat. No. 4,320,525 issued to Woodward on 16 March 1982; and U.S. Pat. No. 4,313,206 issued to Woodward on 26 January 1982.
The early prior art approach to signal recovery or detection is shown in the above-referenced prior art patents. The Ashida patent is directed towards a clock derivation system for deriving clock information from a Manchester code or the like having a base band of twice the clock frequency. The Masek patent illustrates a method for decoding a received bi-phase-coded signal. A signal edge (2) is detected and its polarity is determined in intervals of two periods (2T) for reconstructing the transmitted data. This allows the transmission rate of the signals to be increased up to the maximum frequency limit of the logic family used.
The Woodward patent (U.S. Pat. No. 4,313,206) discloses a circuit arrangement for extracting clocking signals from double frequency waveforms in which transitions from an incoming waveform are detected by comparing the output of a first delay line tap with the incoming waveform and simultaneously comparing the outputs of second and third delay line taps thereby generating first and second waveforms used to form a composite clock.
Another Woodward patent (U.S. Pat. No. 4,320,525) discloses a digital circuit for reconstructing the timing information encoded within digital waveforms of the double frequency class which includes facilities for restoring the circuit to proper synchronization at start up time or after a burst of noise of an incoming waveform.
The present invention is directed to a detection apparatus that decodes bi-phase-L (also called Manchester Code), bi-phase-M or bi-phase-S digital signals. It also recovers the clock and detects certain non-Manchester codes. A description of the different bi-phase-L-phase coding techniques are described in a handbook INTERFACE by Kenneth M. True which was published by Fairchild Semiconductor, 464 Ellis St., Mountain View, Calif. 94042.